Luche reduction is the selective organic reduction of ñ,ò-unsaturated ketones to allylic alcohols. The active reductant is described as "cerium borohydride", which is generated in situ from NaBH<sub>4</sub> and CeCl<sub>3</sub>(H<sub>2</sub>O)<sub>7</sub>.
The Luche reduction can be conducted chemoselectively toward ketone in the presence of aldehydes or towards ñ,ò-unsaturated ketones in the presence of a non-conjugated ketone.
An enone forms an allylic alcohol in a 1,2-addition, and the competing conjugate 1,4-addition is suppressed.
The selectivity can be explained in terms of the HSAB theory: carbonyl groups require hard nucleophiles for 1,2-addition. The hardness of the borohydride is increased by replacing hydride groups with alkoxide groups, a reaction catalyzed by the cerium salt by increasing the electrophilicity of the carbonyl group. This is selective for ketones because they are more Lewis basic.
In one application, a ketone is selectively reduced in the presence of an aldehyde. Actually, in the presence of methanol as solvent, the aldehyde forms a methoxy acetal that is inactive in the reducing conditions.